Engineers with the Nuclear Regulatory Commission have filed a petition with the Nuclear Regulatory Commission to order immediate enforcement actions against licensees of U.S. nuclear power plants.

The flaw is in the original design of the electrical system, and has escaped notice for decades. According to the engineers’ petition, as well as a series of staff analyses on file at the NRC, the design flaw occurs in what is called a “single phase” condition in which little or no electricity is entering the plant to operate its backup cooling systems in the event of a blackout or other event cutting off power from the grid. The result is that the motors of backup generators are underpowered and this can cause their motors to burn out. When that happens, there is no way to keep the reactor core cool.

…the petition states, if the plants are not ordered to immediately redesign their electrical systems then the Commissioners should “issue Orders to immediately shutdown the operating nuclear power plants since the licensees are operating their facilities without addressing the significant design deficiency…and with inoperable electric power systems….”

The situation evolved from an unplanned shutdown in January 30, 2012 in Unit 2 of the Byron Station Nuclear Power Plant in Illinois. At the time, it was thought that the shutdowns resulted from a string of unfortunate coincidences. But further examination by the NRC’s electrical engineering branch found something more alarming.

Alternating current comes out in three currents, or phases, which are positive, negative, and neutral. At the high voltage levels coming directly from the power plant, the currents are on separate lines, labeled A, B, and C. David Lochbaum, nuclear safety expert with the Union of Concerned Scientists, explained that “the output from A and B are constantly monitored to make sure they are together, or in phase.

“There are circuit breakers and sensors within the system noting if there is a fault and the two are not in phase. When that happens, a circuit breaker opens to block that line and reroute the electricity. The grid operates on the same principal, with circuit breakers isolating lines when there are interruptions so the entire Northeast doesn’t have a blackout.

“Within the plant there are electrical breakers signaled to open to isolate the problem and others will close for the systems around it. At Byron that didn’t happen. And they didn’t monitor the phase that failed.”

At Byron, however, the single phase, Line C, was not monitored and, in fact, had broken and fallen to the ground between the plant’s main transformer and the nearby power substation. Unfortunately, the staff analysis stated, the line on the ground “did not result in a detectable ground fault” since single phases were not monitored. Because of this power shortage, none of the plant’s four reactor coolant pumps were operable.

Plants are required to have two separate sets of electrical power lines and monitors for their core cooling systems so that operators can still control the reactor even if one line, or train, is damaged by fire or another event.

The loss of a single phase of alternating current, the NRC staff found, “can potentially damage both trains of the emergency core cooling system.” In that case, there is nothing to prevent a meltdown.

The NRC engineers are petitioning that this problem, which exists in most if not all US nuke plants, be resolved or those plants should be immediately decommissioned. Dave Lochbaum, a nuclear expert at nonprofit group the Union of Concerned Scientists, told Reuters it was encouraging the engineers stepped forward without fear of retribution.

It might seem ironic to worry about a power outage at a power generation facility, but when it comes to nuclear, the irony is long worn out.

An element of the Fukushima disaster that is not widely known was the fact there were two Fukushima plants. Fukushima Daiitchi (number 2) was the sight of the infamous explosion that poisoned a large swath of Japanese countryside and a small city. Fukushima Daiini (number 1) was also damaged in the earthquake and flooded by the following tsunami, but Daiini was able to avoid meltdown thanks to one operating offsite power line. One.

That single offsite power source meant the difference between the most famous nuclear accident in the last decade, and a close call most people never heard about. Now this new evaluation by NRC engineers suggests that faith in such offsite power supply systems are misplaced.